1. Field of the Invention
The present invention relates to an optical information recording medium and a method of the production of organic thin film which is used as an under layer of an optical information recording medium on which pits are formed.
2. Description of the Prior Art
As shown in FIG. 1, there has been known an optical information recording disc having a recording layer 12 made of heat mode recording material, such as low melting point metal and organic dye material, formed on a signal receiving surface 11a of a substrate 11 which is made of transparent material, such as glass or plastic resin material. In this kind of optical information recording disc, when a radiation beam modulated by an information signal is radiated from the side of the substrate 11 onto the recording layer 12, the portion radiated by the radiation beam is partly heated so that there occurs a thermal deformation, such as melting, evaporation and shrinkage in the recording layer 12, and the information signal is recorded in a form of a pit (such as a through hole or groove).
However, in the case when the recording layer 12 made of this kind of recording material mentioned above is formed directly on the substrate 11, the recording sensitivity of the optical information recording disc is lowered and there are problems encountered such that,
(1) a recording light source of high power is needed, and
(2) since the angular velocity of the substrate 11 to the optical head can not be made high, improvement of the recording density and increasing the data transfer speed can not be achieved.
In order to solve the problems mentioned above, the present applicant has previously proposed, in Japanese Patent Application No. 59437/1986, an optical information recording medium having an intermediate layer 13 made of an organic compound which is meltable, decomposable or sublimatable at a lower temperature than the melting, decomposing or sublimation temperature of the recording layer 12, including at least fluorine and carbon, formed between the substrate 11 and the recording layer 12, as shown in FIG. 2.
In the optical information recording medium shown in FIG. 2, since between the substrate 11 and the recording layer 12 there is provided an under layer 13 made of polytetrafluoroethylene (referred to as PTFE hereinafter) having a heat resistance against the temperature better than that against the temperature of the thermal deformation of the uneven pattern formed on the substrate 11, it becomes harder for the heat at the time of the radiation of the laser light beam for playback to be conducted to the surface of the substrate where the uneven pattern is formed, so that thermal deformation of the uneven pattern is prevented and the degree of the decrement of the carrier to noise level (referred to as CN hereinafter) ratio of the read out signal with the lapse of time is lowered. Since the under layer made of PTFE is melted or decomposed at a temperature lower than the melting temperature of the recording layer, the thermal deformation of the recording layer is accelerated so that the recording sensitivity can be improved. Herein, a sputtering method is adopted as a method of forming the middle layer 13.
However, according to a conventional method of sputtering PTFE in fluorocarbon gas, the partial pressure of which is less than 3 % in the vacuum tank, while high quality thin films of high density with few pin holes can be formed, the speed of forming the film is slow (approximately 1.5 .ANG./sec.) and there is a problem that the productivity is low.
Moreover, it is difficult to bond the PTFE of plate shape to a cathode surface with an adhesive and the bonding intensity between the PTFE plate and the cathode surface is so small that the PTFE plate is easily peeled off due to the heating at the time of sputtering. Therefore, the PTFE plate has to be mechanically secured to the cathode by means of screws. However, since the central portion of the PTFE plate, which is not secured to the cathode, is thermally deformed due to the plasma atmosphere and the sputtering processing, the thin film can not be formed effectively.
Furthermore, in the case when the thickness of the PTFE plate is made to be more than 8 mm, the thermal conductivity of the PTFE plate is so small that the PTFE plate becomes easily deformed due to the heat caused by the sputtering processing in the plasma atmosphere.
Though the plasma polymerization method has been known as a method of forming an organic thin film at a high speed, in the case of the plasma polymerization method, since the organic thin film is formed by supplying a lot of monomer into a vacuum tank, there is produced a lot of waste dust of the polymer between the electrodes, therefore, there are problems that the vacuum tank and the environment are contaminated or a pump system is apt to be easily damaged and that a great labor is needed for the maintenance of the device for forming the thin film.
Furthermore, in the optical information recording disc having an under layer formed of PTFE, compared to the optical information recording disc having an under layer of evaporation or plasma polymerization film made of materials such as guanine or hydrocarbon shown in the invention of the previous application, in the case when the playback operation is repeated for ten hours, the value of the laser power at the time of generating a signal fluctuation of .+-.10% is lower (the values of the laser power are respectively 1.5 mW in the case of the guanine under layer and 1.6 mW in the case of the hydrocarbon plasma polymerization film under layer, while on the other hand, 1.3 mW in the case of PTFE under layer) and the signal readily becomes inferior when used for a long period. Moreover, there is a problem that, in order to obtain the aforementioned value by using the under layer made of PTFE, the thickness of the under layer should be made approximately 800 .ANG. (the thickness is 300 .ANG. in the case of the under layers of guanine and hydrocarbon plasma polymerization film), therefore, the productivity of the optical information recording medium is lower compared to those cases when forming the under layer made of guanine or hydrocarbon plasma polymerization film.